The present invention relates to a crystal growth apparatus comprising a heating furnace, which has a multi-stage heater. Particularly, it relates to a crystal growth apparatus of a compound semiconductor that requires a precise temperature control, and to an effective technology applied to a crystal growth method by using the crystal growth apparatus.
Generally, the vertical gradient freezing (VGF) method, vertical Bridgman (VB) method, horizontal gradient freezing (HGF) method and horizontal Bridgman (HB) method or the like have been known as methods for growing a compound semiconductor single crystal. In these methods, a compound semiconductor single crystal is grown by utilizing the temperature gradient in a growth furnace.
In a crystal growth apparatus to which such a growth method is applied, in order to realize the desired temperature gradient, there is a case that a heating furnace having a multi-stage heater is used. As an invention using a heating furnace having a multi-stage heater, there is, for example, the invention described in the International Publication No. WO95/22643. In this invention, by utilizing the VGF method, the temperature distribution in a heating furnace is controlled so that a first vertical temperature gradient in vicinity of the external wall of the quartz ampoule corresponding to a raw material melt may be smaller than a second vertical temperature gradient above the upper end of a crucible. Then, the temperature in the heating furnace is decreased gradually, for the crystal to be grown toward the lower side from a surface of the raw material melt.
Further, in the prior application, in order to control the temperature gradient accurately, a crystal growth apparatus comprising a heating furnace having at least six heating means (heaters), is suggested. A schematically sectional view of the crystal growth apparatus is shown in FIG. 5. In FIG. 5, the crystal growth apparatus 100 comprises a heating furnace 110 having an upper stage heater portion 101 to 104 and a lower stage heater portion 105 to 107, a control device, which controls the furnace internal temperature distribution or the like by controlling electric energy to each heater, and a power source device. A quartz ampoule 111 having a reservoir portion 111A, in which a crucible 112 made of pBN and charged with a raw material 113 is sealed, is disposed in the heating furnace 110. The heating furnace 110 comprises a heater 103, which controls the first temperature gradient by heating the position corresponding to the crucible 112, a heater 102, which controls the second temperature gradient by heating the space above the upper end of the crucible 112, a heater 106, which controls the vapor pressure by heating the reservoir portion 111A, heaters 101 and 107, which suppress the influence of disturbance to the furnace internal temperature distribution, and heaters 104 and 105, which suppress the mutual influence between the heater 103 and the heater 106.
According to the crystal growth apparatus, since the heaters are in multi-stage, it is possible to control extremely satisfactorily the first temperature gradient and the second temperature gradient, and there is the advantage that the yield of a single crystal substrate can be improved rapidly.
However, in growth of a compound semiconductor single crystal by using the above-described crystal growth apparatus having a multi-stage heater, although it is possible to improve the yield of a single crystal substrate, when aiming at an inplane dopant (impurity) concentration of a obtained wafer, it was found by an experiment that there is dispersion at least beyond the measurement error in a plane.
The present inventors repeated eagerly researches about the case. When a crystal is grown by using the above-described crystal growth apparatus, the temperature distribution around the crystal was measured in the same horizontal plane along the circumferential direction, and only one place was evidently low, compared with the others. Then, the temperature distribution was compared with the inplane dopant concentration distribution of the wafer, and it was noticed that the temperature distribution is corresponding to the inplane dopant concentration distribution. Therefore, it is reached to the conclusion that solidifying process of a crystal is shifted in the same plane since the inplane temperature distribution of the heating furnace is not uniform, and consequently, dispersion of the inplane dopant concentration of the wafer occurs.
Furthermore, it is found that in the inplane temperature distribution in the furnace, the region that the temperature is low is corresponding to the position of the terminals of the heaters. Therefore, it is ascertained that heat radiation from the terminal portions of the heaters is the cause of the inplane temperature distribution in the furnace not being uniform. A perspective view of the upper stage heater portions 101 to 104 in FIG. 5, is shown in FIG. 6. The heaters and a power source device or a control device are connected through wirings. However, in the conventional crystal growth apparatus, the terminal portions 101a to 104a, which are taken out from each heater, were always disposed in the same place on the circumference, seeing from the axial direction of the heating furnace, in order to be wired easily. In this case, since there is heat radiation from the terminal portions, the furnace internal temperature in vicinity of the terminal portions was decreased somewhat, so that the temperature distribution in the same horizontal plane in the furnace was dispersed delicately.
The present invention was made to solve the problem. The objects of the invention are to provide a crystal growth apparatus comprising a heating furnace, which can control the temperature distribution in the same horizontal plane in the furnace, and to provide a method for producing a single crystal by using the crystal growth apparatus.
In order to accomplish the objects, in the present invention, a crystal growth apparatus comprises a cylindrical heating furnace having a multi-stage heater, and the inplane temperature distribution in a heating furnace is made to become uniform without gathering terminal portions taken out from each heater in one place.
Concretely, when the number of the heaters is, for example, two, each heater may be disposed so that the terminals may be located at an almost facing position. Further, when the number of the heaters is N (N is a positive integer of three or more), each heater may be disposed so that the terminal portions of the heaters may be located at each apex of a regular n-gon (n is an integer which satisfies 3xe2x89xa6nxe2x89xa6N), seeing from the axial direction of the heating furnace. Therefore, since heat radiation from the terminal portions does not occur at one place, it is possible to uniform the inplane temperature distribution in the heating furnace.
In order to grow a single crystal by using the crystal growth apparatus, for example, a heat-resistant container charged with a raw material is disposed in the heating furnace portion of the crystal growth apparatus. The heater is controlled and a predetermined temperature distribution is made in the furnace. Then, after melting the raw material by heating the heat-resistant container portion to be over the melting point of the raw material, the crystal is grown preferably by decreasing the temperature of the heating furnace gradually, while maintaining the temperature distribution. Thereby, the uniformity of the inplane dopant concentration of a wafer obtained can be improved sharply.
Further, a crystal growth apparatus of a compound semiconductor comprises a heating furnace having an upper stage heater portion heating a crucible portion charged with a raw material of a compound semiconductor, and a lower stage heater portion heating a reservoir portion communicated with a quartz ampoule sealing the crucible. The upper stage heater portion is constituted of N numbers of heaters, which are laminated in multi-stage in an axial direction. Each heater is disposed in order for the terminal portion of each heater in the upper stage heater portion to be located at each apex of a regular N-gon, seeing from the axial direction of the heating furnace. Moreover, each heater may be disposed in order for the terminal portion of each heater in the upper stage heater portion to draw a spiral along the external circumference of the heating furnace. Thereby, the inplane temperature distribution of the raw material portion can be maintained uniform in the upper stage heater portion. Moreover, a volatile element disposed in the reservoir portion is evaporated by heating the reservoir portion in the lower stage heater portion and the pressure in the sealed container can be controlled by the vapor pressure. Additionally, a point defect in the crystal can also be prevented from occurring by volatilization of an easily volatile component.
In order to grow a compound semiconductor single crystal by using the crystal growth apparatus comprising a cylindrical heating furnace having an upper stage heater portion and lower stage heater portion, for example, a crucible charged with a raw material of a compound semiconductor is disposed in a quartz ampoule that has a reservoir portion, and a simple substance or a compound, which consists of at least one kind of volatile element among the constituent elements of the compound semiconductor, is charged into the reservoir portion of the quartz ampoule. Then, the quartz ampoule is disposed in the crystal growth apparatus so that the crucible may be located at inside of the upper stage heater portion and the reservoir portion may be located at inside of the lower stage heater. The upper stage heater portion is controlled and a predetermined temperature distribution is made in the furnace. Then, after melting the raw material of the compound semiconductor by heating the crucible portion over the melting point of the raw material, the volatile element is evaporated by heating the reservoir portion in the lower stage heater portion, so that the pressure in the quartz ampoule is controlled by the vapor pressure. The crystal is grown preferably by decreasing the temperature in the heating furnace, while maintaining the temperature distribution. Therefore, in production of a compound semiconductor single crystal, the inplane dopant concentration of an obtained semiconductor wafer can be improved sharply. Further, a point defect in a crystal can be prevented from occurring by volatilization of an easily volatile component.